This invention relates to dredges particularly but not solely for dredging tin from the sea bed. As offshore mineral reserves get less and less in shallow areas within the range of conventional dredges much time and thought is being applied to other types of dredge that can operate successfully at greater depths. Alluvial areas have been found at depths of 300 ft plus and the dredge of the present invention is intended for operation of up to that depth although in principle it can be applied to much greater depths. A known early type of dredge was powered by steam and was equipped with 24 meters long ladder supporting a band of thirty eight 7.1/4 cu.ft. buckets capable of dredging to 12 meters below waterline. The dredge capacity was rated at 15,000 cu. meters per month. The treatment plant consisted of a trommel screen feeding undersize, which contained the tin ore, over chutes which were interspaced along their lengths with bars, behind which the concentrates lodged. In rotation the chutes (or palongs) would be closed down for workmen to dig out the trapped concentrate which was then transported ashore for washing up and upgrading.
The recovery efficiency of the treatment plant was low but these dredges show the viability of a bucket line dredge, albeit close to shore, for working off-shore alluvial deposits.
Dredges were developed later driven by diesel electric power having bucket sizes ranging from 7 to 22 cu. meters, dredging to depths varying from 18 to 50 meters and throughputs of 130,000/438,000 cu. meters per month.
Another form of known dredge comprises twin grabs delivering to a conventional treatment plant via collecting hoppers, conveyors and chutes. This unit operates at a maximum depth of 36 meters and has a rated throughput capacity of 220,000 cu. meters per month.
The dredge known as a suction cutter dredge is capable of dredging to nearly 25 meters below waterline and has a throughput reported at 320,000 cu. meters per month. During operations the dredge is moored on a spud which is a unique feature for a sea dredge.
The trailing suction dredge has two suction heads, one in use during port traverse and the other in use during starboard traverse and was specially designed for a particular area with a very shallow depth of alluvium although it is capable of dredging up to 25 meters below waterline.
The bucket line dredges working off Phuket Island, Thailand, were designed for operation in sheltered waters. Operators have carried out dredging in sheltered and exposed areas and their policy is to work the exposed areas during that part of the year when conditions are calm or relatively calm and continue in the sheltered areas during the monsoon season.
Many potential off-shore mining areas are being discovered which are exposed and do not have the advantage of natural adjacent shelter. Dredges for these locations, although basically similar to other dredges, have to incorporate features that will enable them to work all year round in all but the most severe conditions. During these periods dredging operations would stop but the dredges must still be capable of riding out the worst weather in a standby attitude.
Consideration has been given to the dredging of exposed relatively deep off-shore areas of the sea bed. For economic reasons it is considered necessary to operate for the major part of the year regardless of the prevailing climatic and sea conditions. Problems arise due to the pitch and heave of the dredger pontoon causing undesirable relative movement between the digging point at the bottom of the bucket ladder and the sea bed being excavated.
Other problems have to be considered and applied to off-shore bucket dredges operating in exposed waters:
(1) Design of basic pontoon and superstructures to withstand maximum wave conditions coupled with gale force wind.
(2) Greater freeboard than that acceptable for dredges working sheltered areas. More extensive bulwark protection around dredge.
(3) Articulation at the bottom, top or both ends of the ladder to ensure contact of the buckets with the digging face as the dredge pitches and heaves and also to reduce the magnitude of the impact loadings between the bottom end of the ladder and the sea bed and the resulting shock reactions at the ladder pivot point.
(4) A dredge mooring system that includes features such as automatic line out/line in measurement, line tension indications, line speed indicators and automatic tensioning of trailing lines. This must not only ensure safety in that the dredge is under full tensional control at all times but must also improve dredge throughput efficiency by permitting correct dredging procedures and techniques to be applied to the maximum advantage by the operator.
The development of the bucket line dredge over the years has basically been in the direction of deeper digging, increased capacity units incorporating technology advances as they have become available. Mechanical components, drive systems, treatment plant equipment and systems, tailings deposition and mooring techniques have all been developed, or have evolved, towards greater operational, mechanical and recovery efficiencies. This development is a continuing process as new dredging units are designed for ever increasing performances and duties.
Depths of 150/180 ft. for bucket dredges is considered the maximum possible. Beyond that range of depths limiting factors apply such as the physical sizes and weights of bucket line components, induced loadings in the bucket chain assembly which makes dredging operations more difficult and maintenance arduous and time consuming. It is important for financial viability that a dredge operates for a high percentage of available time and that downtime is kept to a minimum. As depths get greater operating time tends to get less and downtime greater.
The quests for methods of economically dredging off-shore deposits located at greater depths than can be reached by even the present day largest bucket or suction dredges continues to afford designers considerable technical difficulties.
One proposal put forward to meet the challenge was a scheme in which excavation would be effected by means of a continuous chain of dragline buckets passing under a pontoon hull while the dredge traverses similar to a conventional bucket dredge. Each bucket is suspended separately from two parallel catenary lines allowing, to some extent, movement independent of the neighbouring buckets. The profile of the catenary and depth of excavation is varied by altering the position of a submersed frame carrying an idler at the lower end around which the buckets pass. Above this idler at deck level is the drive unit for hauling the catenary lines. In order to maintain an optimum catenary profile the drive unit is capable of longitudinal movement on the pontoon. The buckets are raised over a tipping idler at one end of the dredge where the spoil is discharged into a collection hopper and thence to the various stages of on-board treatment.
Another proposal envisages an arrangement in which two 15 ton grabs would be used for operation in water depths up to 76 meters. A feature of this proposal is the use of a carriage for transporting each grab from the outboard digging position up an inclined tract to the final dumping position over an inboard hopper. The carriage also acts as a movable mounting for the jib-head sheaves. Transition of the grabs from vertical hoisting to the inclined tract is automatic and allows continuous operation without any slewing or luffing motion. Specially designed high pressure water jets traversing a grizzley above the receiving hopper were proposed as the method of breaking up any large clay lumps deposited from the grabs before entering the onboard treatment plant.
Whilst these proposals may well be feasible it is evident that there are many technical problems to be overcome. Furthermore, they would be costly to construct. It is therefore an object of the invention to provide a dredge which is economical to construct and yet overcomes many or all of the technical problems referred to above. It is also an object of the invention to provide an arrangement by which dredged material can be fed at a constant rate to a treatment plant to achieve maximum recovery efficiency. It is envisaged that dredging can be effected at depths well in excess of 100 meters.